1. Field of the Invention
The present invention is broadly concerned with improved, relatively low cost, synthetic resin-based arc-quenching fuse link tubes adapted for use with electrical cutouts or other similar equipment and which serve, under fault current-induced arcing conditions when the fuse link melts, to suppress the arc and thereby clear the fault. More particularly, it is concerned with such improved arc-quenching fuse tubes which include an inner wall segment formed of arc-quenching material, preferably comprised of an epoxy synthetic resin formulation, e.g. bis-phenol epoxy (BPA) or cycloaliphatic epoxy impregnated with an inorganic filler which generates molecular water upon being subjected to arcing conditions. The epoxy matrix is reinforced by provision of an organic fiber such as polyester, rayon or mixtures thereof that supports the resin during cure and contributes to arc interruption. The synthetic resin-based fuse tubes in accordance with the invention completely eliminate the use of conventional bone fiber as a lining material for fuse tubes, while at the same time giving equivalent or even enhanced arc-quenching results, as compared with bone fiber.
2. Description of the Prior Art
The use of so-called bone fiber as a lining material for expulsion fuse tubes is well-established. The arc-interrupting operation of bone fiber in this context results from the fact that the material is a high density, cellulosic, exceptionally strong, resilient material which becomes a charring ablator in the presence of an electric arc. As bone fiber decomposes under the intense arc heat, a char of carbonaceous material is formed in the tube, along with simultaneous production of a number of insulating and cooling gases. The exceptionally low thermal conductivity of the char layer protects the virgin bone fiber from excessive ablation hence rendering the tube reusable. The bone fiber is also somewhat hydrophilic in nature and the adsorbed water is also subject to decomposition to provide gaseous arc-interrupting products. The presence of the evolved gases, along with their turbulent intermixing with the arc, usually leads to a successful circuit interruption. It has been reported that over 90% of the decomposition gases from bone fiber consist of hydrogen and carbon monoxide. These materials are formed by a highly endothermic reaction of carbon with the water present in the bone fiber. Hence, it will be appreciated that the water content of the bone fiber not only provides endothermic cooling by evaporation, but also reacts with carbon to form arc extinguishing gases in the form of carbon monoxide and hydrogen.
As noted, an important characteristic of bone fiber is its tendency to absorb water; however, if atmospheric conditions are either too dry or too humid, the interrupting capability of bone fiber may be adversely affected. Hence, bone fiber is subject to an inherent variability depending in large measure upon uncontrollable ambient conditions.
The carbonaceous char formed when bone fiber interrupts an arc also acts as a thermal barrier to prevent excessive ablation of the bone fiber surface. Such ablation is controlled to a certain extent by the endothermic events associated with the presence of a significant quantity of water, i.e., evaporation and reaction with carbon The carbonaceous char layer must not, however, be too heavy or it will cause a restrike. As the moisture content in bone fiber goes down, more of the arcing energy is available for char formation, and hence the probability of a restrike increases.
While the use and operational efficiency of bone fiber is thus well known, a number of severe problems remain. In the first place, bone fiber is in short supply; only two reliable remain in the market and how long they will continue to do so is unknown The material is difficult and time-consuming to make, and therefore is costly. Furthermore, it is produced only in certain standard lengths, and this inevitably means that there is substantial wastage when the tube lengths are cut for tube fabrication purposes.
In addition, a completed fuse tube employing bone fiber typically comprises an outer synthetic resin reinforced shell with the bone fiber secured to the inner portions thereof as a liner. It is sometimes very difficult to properly adhere the bone fiber to the outer shell, and in most cases a weak mechanical bond is the best that can be accomplished.
Finally, it has been established that the expulsion forces generated by bone fiber during an arc interruption are considerable, and this in turn requires that the fuse assembly hardware holding the tube be relatively massive and hence expensive.
All of these drawbacks have made it clear that there is a real need in the art for an adequate replacement for bone fiber in the construction of arc-quenching fuse tubes.
Although there have been prior efforts to provide synthetic resin substitutes for bone fiber fuse tubes, the solutions suggested have not found significant commercial acceptance.
Canadian General Electric Company Limited has addressed the problem as discussed in Mattuck and Conte U.S. Pat. Nos. 4,373,555 and Bergh U.S. Pat. No. 4,373,556. Both patents describe a cutout fuse tube in which a cycloaliphatic epoxy core is reinforced with polyester fiber to aid in arcquenching. The polyester fiber content is described as being at least about 45% by weight. The outer shell is either a cycloaliphatic or BPA epoxy formulation. Mattuck, et al. suggest that heat treating the polyester fiber may improve the mechanical and electrical characteristics of a fuse tube. The '555 patent to Mattuck, et al. also indicates that the composition may contain aluminum trihydrate (ATH) as a flame retardant. The concentration of ATH is limited to no more than about 15% by weight based on the minimum resin and polyester constituents that must be provided to satisfy the requirements of the patentees' system. Although described as a flame retardant, ATH at that concentration would have very limited flame suppression characteristics and would contribute very little, if any, to arc extinguishment.
Tobin in U.S. Pat. No. 4,349,803 discusses a fuse tube made from a cycloaliphatic epoxy resin that incorporates a melamine or dicyandiamide as an arc-extinguishing filler. Also the patentee teaches reinforcing the interface between the core and shell glass cloth, mat or strands under conditions such that the core and shell agents are said to flow into the reinforcing material during pressure gelation.